Interferometrically operating optical measuring systems including a modulation interferometer and a reference interferometer situated downstream from it are known for the optical detection of the shape, diameter and roughness of surfaces. They are used for optical distance measurement, for example, in quality control during the measurement of surface geometries. EP 1 058 812 B1 describes such an interferometric measuring device for detecting the shape or the distance of in particular rough surfaces, including at least one spatially coherent beam generation unit, whose radiation is split in a measuring probe of the measuring device into a reference measuring beam, which is guided through a measuring reference arm and reflected therein, and a measuring beam, which is guided through a measuring arm and reflected on the rough surface, including a unit for modulating the light phase or for shifting the light frequency according to a heterodyne frequency of a first partial beam in relation to the light phase or the light frequency of a second partial beam, including a superposition unit for superposing the reflected measuring reference beam on the reflected measuring beam, including a beam decomposition and beam reception unit for splitting the superposed radiation into at least two beams having different wavelengths and converting the radiation into electrical signals, and including an evaluation device, in which the shape or the distance of the rough surface is determinable based on a phase difference of the electrical signals. It is provided that the radiation emitted by the beam generation unit is short-coherent and broadband with respect to time, that the beam generation unit, a beam splitter for forming the first and second partial beams, and the unit for phase modulation or frequency shifting are situated in a modular unit, which is spatially spaced apart from the measuring probe and is configured as a modulation interferometer, and that a delay element is situated in the modular unit in the beam path of a partial beam, which results in a difference of the optical path lengths of the two partial beams, which is longer than the coherence length of the radiation emitted by the beam generation unit.
Such interferometric measuring devices, which initially include two interferometers, may be constructed using different types of interferometers. Thus, the modulation interferometer may be constructed as a Mach-Zehnder interferometer, while the measuring interferometer or the measuring probe is constructed compactly, for example, as a Mirau interferometer. The interferometric measuring devices share the feature that a path difference registered in the first interferometer between two partial beams of a short-coherent radiation source is equalized again in the second measuring interferometer or the measuring probe and the partial beams may thus be moved into interference formation. The optical path difference registered by a delay element in EP 1 058 812 B1 may be generated by partial arms of different lengths, through which the partial beams pass, as represented in EP 1 058 812 B1 in a modulation interferometer constructed using optical fibers.
The optical path difference to be registered in the modulation interferometer depends on the structural configuration of the measuring interferometer or the measuring probe.
The publication EP 1 082 580 B1 discusses an interferometric measuring device for detecting the shape, roughness or distance of surfaces, including a modulation interferometer having a spatially coherent beam source and a first beam splitter for splitting its beam into two partial beams, one of which is shifted in its light phase or light frequency with respect to the other with the aid of a modulation device, and subsequently the two partial beams are combined, including a measuring probe in which the combined partial beams are split into a measuring beam guided through a measuring arm and reflected on the surface and a reference beam guided through and reflected in a reference arm, and in which the reflected reference beam is superposed on the reflected measuring beam, and including a receiving unit for splitting the superposed radiation into at least two beams having different wavelengths and converting the radiation into electrical signals, and for evaluating the signals based on a phase difference. It is provided that the modulation interferometer configured as a modular unit is spatially separated from the measuring probe and may be coupled to it via a fiber optic system, and that the measuring arm and the reference arm are formed by one or multiple solid bodies conducting the measuring beam and the reference beam.
The publication DE 197 21 843 C1 discusses an interferometric measuring device for measuring the shape of rough surfaces of a measuring object, having a radiation-generating unit for emitting short-coherent radiation, a first beam splitter for forming a first and a second partial beam, of which the first is directed onto the surface to be measured and the second is directed onto a device having a reflecting element for periodically changing the light path, having a superposition element on which the radiation coming from the measuring surface and the device is made to interfere, and a photodetector device which receives the interfered radiation and sends corresponding electrical signals to a control unit for evaluation. It is provided that the first partial beam is split into at least two additional partial beams with the aid of at least one additional beam splitter, that one of the additional partial beams is guided as a reference partial beam onto a reference mirror situated at a predefined distance from the additional beam splitter, while the at least one other additional partial beam is guided as a measuring partial beam onto a particular measuring point (MP1, MP2) of the measuring object, and that the interference maximums (ER, E1, E2) of the reference partial beam and of the at least one measuring partial beam are separately detectable with the aid of the photodetector device and the control unit. Advantageously, an additional second measuring partial beam is decoupled. By comparing the interference formation of the first measuring partial beam and the reference partial beam with the second partial beam coming from the device, it is possible to obtain a reference point for the first measuring point on the surface of the measuring object. By varying the optical path length in the modulation interferometer, it is possible to shift the scan point of the second measuring partial beam to the second measuring point of the surface of the measuring object. It is thus possible to infer the distance between the two measuring points, for example, the diameter of a drilled hole, from the variation of the optical path length. It is disadvantageous in this case that the measuring zone is limited to the setting possibility of the optical path length in the modulation interferometer, and that a complex beam guidance is required for providing the reference partial beam.
In addition to the described modulation interferometers, further optical, point-measuring scanning systems and measuring methods for determining surface contours and roughnesses are believed to be understood, for example, autofocus sensors, triangulation, chromatic sensors, and white light interferometers.